U.S. Pat. Nos. 2,362,777 and 3,123,007 disclose early systems for hydraulically driving a reciprocating well pump but neither have bearing on the present invention. Many similar patents exist, some having fluid motors for attachment to conventional pumps or to operate a string of sucker rods which in turn operate a conventional downhole pump.
Coberly U.S. Pat. No. 2,952,212, the type pump that virtually all downhole hydraulically driven pumps in use today comprise, operates by co-mingling spent power fluid with produced liquid from the well which requires separation and purification of all of the power fluid before recirculation to the downhole pump. A later Coberly U.S. Pat. No. 3,005,414, employs a power fluid string and a separate conduit to return exhaust power fluid to the surface and a production string to convey produced liquid to the wellhead as does the present invention. However, frequent clogging of small flow paths in that type of pump by solids entrained in the power fluid, require frequent placement of the pump.
As with all hydraulic equipment, some leakage may occur at moving seals, at pipe fittings or the like such that makeup fluid is required to replace such leakage so as to allow for continuous operation of the system. The open system described by Coberly causes co-mingling of the exhaust power fluid with the produced well fluid which in many wells may be mostly water, thereby requiring a production separator to process the entire stream of produced fluid mixed with exhausted power fluid so as to make available power fluid for continued operation. A closed system does not mingle produced fluid with exhausted power fluid and therefore, it is not required to separate the two for each cycle of the fluid, however, makeup fluid is required by the system to offset aforementioned leakage. Makeup fluid for a closed hydraulic system is usually provided by the operator keeping track of oil level in the tank and periodically delivering oil to inject into the system. Unattended operations in the field can thereby be jeopardized by any delay of that service. It is therefore desirable to prevent the need for and cost of separating all power fluid for each cycle and to have makeup power fluid automatically supplied to the system without the need for extended supply lines from distant tanks and/or treatment facilities, or the manual replenishment of oil to the system.
Presley U.S. Pat. No. 4,233,154 discloses a production separator, normally mounted a considerable distance from the wellhead and in some cases miles from the wellhead, that makes provision to pump fluid back to the well for driving a downhole pump in an open system as taught by Coberly U.S. Pat. No. 2,952,212. No prior art known to the inventor provides for a downhole filter so as to exclude particles, within the power conduit being used to convey pressurized fluid downwardly to the pump motor, from entering the pump motor; automatically provides at the wellhead, for the proper volume of makeup fluid to offset leakage from a closed system; prevents precipitation of paraffin from the well fluid by heating power fluid used to drive a downhole pump; eliminates the hydrostatic pressure differential across the wall of the conduit used to convey pressurized power fluid. Such omissions may partially explain the extremely limited application to date of hydraulically driven downhole pumps.